The present disclosure relates to exhaust gas sensors. More particularly, the present disclosure relates to an exhaust gas sensor with enhanced nitrous oxides sensing capabilities.
Exhaust sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust sensors have been used for many years in automotive vehicles to sense the presence of exhaust gases. In automotive applications, the direct relationship between various exhaust gas concentrations and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the sensor to provide concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
Particularly with nitrogen oxides (NOx), there are several different ways to detect NOx in exhaust gas. These methods are thermal, optical, electronic resistive, and electrochemical. U.S. Pat. No. 5,486,336 to Betta et al., U.S. Pat. No. 4,822,564 to Howard, U.S. Pat. No. 5,800,783 to Nanaumi et al., and U.S. Pat. No. 4,927,517 to Mizutani et al. demonstrate each of these methods of detecting NOx, respectively. Among the conventional NOx detection methods, the electrochemical method has proven to be particularly effective because the sensor materials are compatible with the high temperature environment created by the exhaust gas. With the electrochemical method, there are two basic principles involved in NOx sensing: the Nernst principle and the polarographic principle.
With the Nernst principle, chemical energy is converted into electromotive force (emf). A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to the partial pressure of a known gas. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine""s exhaust. This is particularly relevant as NOx sensors catalytically reduce NOx to nitrogen gas and oxygen, wherein the liberated oxygen is then measured. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:       E    =                  (                              -            RT                                4            ⁢            F                          )            ⁢              xe2x80x83            ⁢      ln      ⁢              xe2x80x83            ⁢              (                              P                          O              2                        ref                                P                          O              2                                      )                  where:                                E          =                      electromotive            ⁢                          xe2x80x83                        ⁢            force            ⁢                          xe2x80x83                        ⁢                          (              emf              )                                                                    R          =                      universal            ⁢                          xe2x80x83                        ⁢            gas            ⁢                          xe2x80x83                        ⁢            constant                                                        F          =                      Faraday            ⁢                          xe2x80x83                        ⁢            constant                                                        T          =                      absolute            ⁢                          xe2x80x83                        ⁢            temperature            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                        ref                    =                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            reference            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                                =                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            exhaust            ⁢                          xe2x80x83                        ⁢            gas                              
With the polarographic principle, the sensors utilize electrolysis; that is, by measuring the current required to decompose a gas, such as NOx, the concentration of that gas can be determined. Generally, this type of sensor is composed of a pair of current pumping electrodes where both are in contact with an oxide conductive solid electrolyte and one electrode is in contact with a gas diffusion limiting medium. The gas diffusion limiting means in conjunction with the pump electrode creates a limiting current which is linearly proportional to the measured gas concentration in the sample.
For example, one known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner. Within the sensor, a flat plate sensing element is employed. This sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of brittle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. This is particularly problematic since most materials conventionally used as sensing element supports, for example, glass and ceramics, cannot withstand much bending. After the sensor is formed, exhaust gas can be sensed.
Particular to NOx sensors, treatment of the exhaust gas is employed prior to being analyzed utilizing the Nernst and/or polarographic principles. Typically, this is achieved using catalyst and/or by maintaining the other gasses at constant levels within an enclosed or semi-enclosed environment. Once the exhaust is treated, the gas encounters the sensor""s electrochemical cells.
A typical prior art NOx sensor will have two electrochemical cells. The first cell has an exhaust gas diffusion limiting means, two oxygen pumping electrodes, and two oxygen sensing electrodes separated by an oxide conducting solid electrolyte. The second cell has a gas diffusion limiting means that connects to the first cell, two pumping electrodes, two sensing electrodes, and an oxide conducting solid electrolyte between the electrodes. The first cell has one pumping electrode exposed to ambient exhaust gas and the other pumping electrode exposed to the inside of the first cell. As to the first cell""s sensing electrodes, one is exposed to a reference gas while the other is located within an interior portion of the first cell. The pumping electrodes of the second cell have one electrode exposed to exhaust gas and the other electrode exposed to the interior of the second cell. As with the first cell, the second cell has one sensing electrode exposed to a gas and the other exposed to the interior of the second cell. In use, the electrodes located inside the first cell have substantially no effect on the NOx concentration so that only the oxygen concentration is modulated and not the NOx concentration. The electrodes inside the second cell have an effect on the NOx concentration via using a catalyst. Thereby, NOx sensing can be achieved with either the Nernst and/or the polarographic principles. Generally, a heater is provided to maintain a constant operating temperature within the sensor.
As such, existing electrochemical NOx sensors employ multiple electrochemical cells that share a common oxide conducting solid electrolyte. These cells have a frequent tendency to electrically cross-communicate and interfere with each other. Accordingly, there remains a need in the art for a NOx sensor having minimal cross-communication and interference between sensor electrochemical cells.
The deficiencies of the above-discussed prior art are overcome or alleviated by the gas sensor and method of making the same. One embodiment of the gas sensor comprises: a first electrochemical cell having a first electrolyte disposed between and in ionic communication with first and second electrodes; a second electrochemical cell having a second electrolyte disposed between and in ionic communication with third and fourth electrodes wherein said first and second electrochemical cells are ionically isolated from each other; and a third electrochemical cell having a fifth electrode disposed on the same side of the second electrolyte as the third electrode. The fifth electrode and third electrode are arranged to be disposed in a spaced relation. Additionally, the first and second electrolytes are each disposed in a separate layers of dielectric material.
In another embodiment, the gas sensor, comprises: a first electrochemical cell having a first electrolyte disposed between and in ionic communication with first and second electrodes; a second electrochemical cell having a second electrolyte disposed between and in ionic communication with third and fourth electrodes wherein said first and second electrochemical cells are ionically isolated from each other; a third electrochemical cell having a fifth electrode disposed on the same side of the second electrolyte as the third electrode, wherein the third and fifth electrodes are disposed in a spaced relation; and a fourth electrochemical cell disposed on a side of the second electrochemical cell opposite the first electrochemical cell, the fourth electrochemical cell having a third electrolyte disposed between and in ionic communication with sixth and seventh electrodes, wherein the fourth electrochemical cell is disposed in a dielectric layer, and wherein the first, second, and fourth electrochemical cells substantially ionically isolated from one another. The first and second electrolytes are each disposed in a separate layers of dielectric material.
Alternatively to avoid electrical cross-communication and interference between electrochemical cells using a shared electrolyte, a dielectric insulation layer is inserted between the two electrochemical cells. Or to avoid electrical cross-communication and interference between electrochemical cells, the cells arrangement involve both of the two schemes mentioned above.